A study of the time-domain topology energy based ultrasonic Lamb wave imaging method for multi-blind hole defects

The traditional ultrasonic imaging methods are constrained by Rayleigh criterion, so it is difficult to image multiple defects in the case of defect spacing less than the imaging resolution threshold. In this paper, an ultrasonic Lamb wave imaging method based on time-domain topological energy is proposed, which converted the topological gradual process in the inverse scattering topological imaging method into solving the forward sound field and the adjoint sound field. Then, the two sound fields, in which the adjoint sound field is time reversed, have the characteristics of focusing at the defects rather than focusing in the non-defective area. By fusing the forward sound field and the adjoint sound field and using the time-domain topological energy value as the pixel value for imaging, the defect can be characterized with high precision. In this paper, the finite element models of multi-blind hole defects in the case of defect spacing less than the imaging resolution threshold is established. The S0 mode Lamb wave is excited and the defect scattering signals are collected in the way of "one transmitter and multiple receiver" for time-domain topological energy imaging. The simulation results show that for multi-blind hole defects, the time domain topological energy imaging (TDTE) method can obtain higher resolution than the delay and sum (DAS) method and the time reversal (TR) imaging method, overcome the influence of mode conversion and achieve high-resolution imaging in the case of the defect spacing less than the imaging resolution threshold.